Rapid evolution of material systems and continued tightening of quality control constraints for thin-film manufacturing processes in semiconductor and other (e.g., optical coating) industries pose a number of challenges to equipment design, giving rise to a wide range of reactor systems designed to reduce spatial non-uniformity of deposition thickness, composition, microstructure and other quality parameters of thin films. In some manufacturing processes, the use of substrate (wafer) rotation is integral to achieving acceptable film properties across the substrate. In Chemical Vapor Deposition (CVD) systems commonly used for semiconductor processing numerous reactor designs make use of wafer rotation, such as
1. cross-flow reactor designs shown in FIG. 1A, where gas flows through a tube or duct-shaped reactor chamber over a wafer and exhausts opposite the gas inlet where wafer rotation is used to reduce cross-flow deposition non-uniformities and depletion effects in the direction of flow;
2. cylindrical reactors shown in FIG. 1B, in which gas flows from a shower head over a wafer and exhausts from the bottom, where wafer rotation is used to eliminate any residual angular non-uniformities in the reactor design; and
3. planetary reactors shown in FIGS. 1C-1D, where gas flows radially outward from a central feed point over a susceptor containing multiple wafers. As the gas flows over the hot susceptor, thermal decomposition and gas phase and surface reactions take place, some of which result in film deposition of the desired material, while other reactions lead to the formation of gas-phase adducts and deposits on reactor walls and other surfaces.
Due to depletion and the manner in which decomposition reactions take place in this reactor geometry, radial flow designs inherently produce non-uniform deposition patterns with respect to the radial coordinate of the reactor. In an effort to overcome this limitation, some reactor designs incorporate a substrate planetary motion mechanism as part of the susceptor assembly to compensate for the depletion and other radial and azimuthal variations in CVD reactor systems as shown in FIG. 1C. In these reactors, the relatively large susceptor 10 rotates around its center point and each wafer 12 rotates independently of the susceptor. This combination of rotating motions results in points on each wafer tracing out a cycloid-like pattern partially compensating for the non-uniform deposition profile. Typically, these reactors can be operated with rotating wafers or stalled (non-rotating) wafers. This design has the effect of eliminating reactor-induced angular non-uniformity generators through susceptor rotation. Wafer rotation is used to reduce the intrinsic (and completely unavoidable) effect of gas phase reactant decomposition and precursor depletion in the gas phase.
The physical and electrical properties of SiC and group-III nitrides (e.g., AlN, GaN, InN, and their alloys) make these materials ideal for high-frequency, high-power electronic devices as well as optoelectronic applications. For example, gallium nitride (GaN), a compound semiconductor material, has shown potential in electronic and optoelectronic devices over the past few years due to its wide-bandgap and high breakdown field properties. GaN has a direct bandgap of 3.4 eV making it suitable for manufacturing light emitting diodes (LEDs) capable of emitting light of any wavelength between blue and ultraviolet (UV) when alloyed with indium (In) and aluminum (Al). In addition, GaN-based devices are used for high-frequency and/or high-power applications including aircraft radar electronics.
Metalorganic vapor phase epitaxy (MOVPE) is the principal method used to grow single-crystalline layers of this material. Currently, manufacturers of gallium nitride devices use both commercial and custom-built reactor designs. The wide range of reactor designs indicates a lack of a coherent framework on how to design gallium nitride reactors for optimal single wafer and multiple-wafer production. As a result, significant research from both academic and industrial levels has enhanced manufacturing technology considerably within the past decade.
Despite ongoing research in this area, an unambiguous understanding of the physical and chemical mechanisms governing the deposition process is still lacking. The difficulties in achieving this understanding to a certain extent can be linked to the complex intrinsic chemistry of the deposition process, the knowledge of which currently is incomplete. A large number of gas phase and surface phase reactions resulting from the extreme conditions necessary for gallium nitride growth have been extensively studied by many researchers. As a result, a number of chemical mechanisms describing important gas phase and surface phase reactions during GaN growth have been reported in the literature. Though most of these mechanisms present similar reaction pathways, the distinguishing factors are the individual rate parameters. In addition, some research groups assume significant gas phase reactions, whereas others assume gas phase reactions play no role in film deposition kinetics. A consensus of a definitive kinetic model describing gallium nitride growth has yet to be reached.
Gas phase gallium nitride chemistry may be visualized as consisting of two competing routes: an a) upper route and b) lower route. The upper route is more commonly referred to as the adduct formation pathway, whereas, the lower route refers to the thermal decomposition pathway of TMG (Trimethylgallium). Each pathway is responsible for producing an array of chemical species that may eventually participate in GaN deposition. The primary gas phase reaction is the spontaneous interaction between commonly used precursors, trimethylgallium ((CH3)3Ga) and ammonia (NH3), to form stable acid-Lewis base adducts. Adduct formation is a ubiquitous problem during MOVPE of GaN and has been widely studied. Upon formation, these adducts may condense on cold surfaces inside the reactor system. For this reason, the formation of these adducts is believed to degrade film quality, uniformity, and consume the feed stream of organometallic sources.
Consequently, numerous research groups have designed reactor systems, in particular gas delivery systems, with the intent to minimize precursor interactions. The most common approach has been to use separate injectors to reduce any premature mixing of the precursors. Reactor systems of this type have been developed by SUNY/Sandia/Thomas Swann researchers to illustrate a connection between gas phase reactions and film-thickness uniformity. It should be noted that while these designs are able to suppress reactions in the gas delivery system, complete mixing of the precursors must take place close to the wafer surface to achieve uniform film thickness.
More novel approaches to the optimization of GaN CVD are based on optimization of two objective functions that span multiple length scales which are performed simultaneously to maximize thickness uniformity (macroscopic objective) and minimize surface roughness across wafer surface (microscopic objective). Additionally, a strategy for nonlinear programming problems that involved PDE models has been developed and applied to a detailed GaN CVD model where the objective was to optimize operating conditions that produced thin films of GaN with spatially uniform thickness.
While a number of simulation-based optimization studies have been performed on existing reactor systems, none of them however have addressed a fundamental question of whether the non-uniform deposition profiles exist in the reactor radial coordinate which produce perfectly flat deposition profiles on the rotated wafers.